Problem: Jessica is 4 times as old as Luis and is also 21 years older than Luis. How old is Luis?
Answer: We can use the given information to write down two equations that describe the ages of Jessica and Luis. Let Jessica's current age be $j$ and Luis's current age be $l$ $j = 4l$ $j = l + 21$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $l$ , and both of our equations have $j$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $4l$ $-$ $ (l + 21)$ which combines the information about $l$ from both of our original equations. Solving for $l$ , we get: $3 l = 21$ $l = 7$.